The present invention broadly relates to frequency sweeps, spectrum analyzers and demodulators, and deals more particularly with a surface acoustic wave demodulator employing corrective feedback that compensates for the nonlinear effects on signal processing which are caused by analog devices employed to effect frequency dispersion.
Analysis of signals in the frequency domain is widely used in communications systems as well as to obtain physical and electrical system performance information. For example, manufacturers of mechanical structures, such as aircraft and bridges, may perform spectrum analysis of a signal produced by a motion-to-electrical signal transducer to permit monitoring of vibration components associated with imbalance and worn bearings and gears, as well as to identify the natural mode of vibration of a mechanical system.
Spectrum analyzers are also used in electronic testing to assess nonlinear effects of amplification, mixing and filtering, to determine the purity of signals, measurement of radio frequency power, frequency and modulation characteristics, and to provide amplitude analysis for electrical networks. In telecommunication systems, transceivers and multiplex systems are assessed with respect to their spectrum, modulation, wave and audio characteristics.
A spectrum analyzer or sweep generator may be employed as a compressive receiver for determining the spectral components of a radio frequency (rf) signal. A communications system, for example, may incorporate such a spectrum analyzer as a demodulator for transforming a frequency division multiplexed (FDM) signal to a time division multiplexed (TDM) signal. Spectrum analysis can be performed digitally by so called "Fast Fourier Transformers" (FFT's) which provide a Fourier transformation of the incoming signal. For some applications, however the computational power requirements for these devices for a desired input bandwidth are impractical. This is particularly true in satellite communications where system power and weight are severely limited. Additionally, the digital processing introduces undesirable delays in the signal, due substantially to the analog-to-digital conversion process which is employed by these devices. Additionally, the bandwidth of some of these devices is limited by the system clock rate.
Analog spectrum analyzers offer the prospect of real time transformations without great demands on processing power. Previous analog spectrum analyzers employed as demodulators have been provided which include a pair of frequency.times.delay dispersion sections with an intervening frequency sweep section. The first dispersion section introduces delays as a function of frequency to an incoming signal, such as a FDM signal. The frequency sweep signal converts the frequency components of the dispersed FDM signal into a series of sweeps. The second dispersion section collapses each sweep into a pulse so that the series of sweeps becomes a TDM signal.
Previous analog spectrum analyzers either employ filter banks or sweep filters. Filter banks are rather bulky and sweep filters are unsuitable for some applications such as demodulators where it is necessary to sample each frequency continuously. The dispersion sections in demodulators of the type described above include devices generically classified as dispersive filters. These filters are also known as surface acoustic wave (SAW) dispersive or linear frequency modulator chirp filters. The first such devices were based on interdigital electrotransducers (IDT's). The IDT consists of a set of interleafed metal electrodes deposited on a surface of a set of piezoelectric substrate, normally quartz. However, IDT based spectrum analyzers have not provided the time-bandwidth product sufficient for some satellite communication applications.
More recently, devices with greater time-bandwidth products have been provided using reflective array compressors (RAC's). The RAC can be manufactured by etching into a crytalline substrate, e.g. lithium nibate, a multitude of slits, e.g. 1000, each tuned to reflect a given frequency. By reflecting different frequencies at different slits, and thus different locations, differential delays are introduced into a through-going signal as a function of frequencies.
The use of RAC's and other similar analog devices in previous types of demodulators introduce error into the processed signal, which in turn limits the effective bandwidth of the receiver. In some cases, the nonlinearities result in phase errors and distorted pulse development. These nonlinear effects are further exacerbated by changes in temperature which alter the operating characteristics of the analog components.